IP = An Internet Protocol;the rules and encoding specifications for sending data.
Your information. Once someone has this they can trace you, find out where you live, who provides your internet, etc. This is also a number than companies can ban from their website, their game, their forums, anything really. Once you get on a site the owner knows your IP, and once it's banned (unless it's dynamic, or you get your static changed) you cannot trick the system by making another account.
But of course, just change your IP, request your ISP to change your IP, or proxy, of course.
Anything else?